A typical long-range optical transmission system includes a pair of unidirectional optical fibers that support optical signals traveling in opposite directions. An optical signal is attenuated over long distances. Therefore, the optical fibers typically include multiple repeaters that are spaced apart from one another. The repeaters include optical amplifiers that amplify the incoming, attenuated optical signals. The repeaters also include an optical isolator that limits the propagation of the optical signal to a single direction.
In long-range optical transmission systems it is important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable such as attenuation in the optical fiber and splice loss, faulty repeaters or amplifiers or other problems with the system. Optical time domain reflectometry (OTDR) is one technique used to remotely detect faults in optical transmission systems. In OTDR, an optical pulse is launched into an optical fiber and backscattered signals returning to the launch end are monitored. In the event that there are discontinuities such as faults or splices in the fiber, the amount of backscattering generally changes and such change is detected in the monitored signals. Since backscatterring and reflection also occurs from elements such as couplers, the monitored OTDR signals are usually compared with a reference record, new peaks and other changes in the monitored signal level being indicative of changes in the fiber path, normally indicating a fault. The time between pulse launch and receipt of a backscattered signal is proportional to the distance along the fiber to the source of the backscattering, thus allowing the fault to be located. In a WDM system, one wavelength is usually assigned as the OTDR channel.
FIG. 1 shows a simplified block diagram of a wavelength division multiplexed (WDM) transmission system that employs a conventional OTDR. Each terminal 110 and 120 includes an OTDR unit 105 and 107, respectively. In operation, OTDR unit 105 generates an optical pulse that is launched into optical fiber 106. The optical pulse serves as the OTDR probe signal. Because optical isolators 115 located downstream from each optical amplifier 112 prevent the OTDR probe signal from being reflected and backscattered to the OTDR 105 on fiber 106, each repeater 114 includes a coupler arrangement providing an optical path for use by the OTDR. In particular, signals generated by reflection and scattering of the probe signal on fiber 106 between adjacent repeaters enter coupler 118 and are coupled onto the opposite-going fiber 108 via coupler 122. The OTDR signal then travels along with the data on optical fiber 108. OTDR 107 operates in a similar manner to generate OTRD signals that are reflected and scattered on fiber 108 so that they are returned to OTDR 107 along optical fiber 106.
FIG. 2 is a block diagram showing one example of a conventional OTDR unit that may serve as OTDR units 105 and 107. The OTDR unit includes a timing generator 211, a light source 212, a detector 214, an amplifier 215, an A/D converter 216, an adder 217 and controller 218. An optical pulse emitted by light source 212, which is driven by a signal from the timing generator 211, is launched into the transmission fiber 106. The reflected and backscattered OTDR signal is received by the detector 214 through a filter 213, amplified with a predetermined amplification factor by the amplifier 215 and introduced to the A/D converter 216. The A/D converter 216 samples the output of the amplifier 215 in a predetermined sampling cycle, and each of the sampled data is supplied to the adder 217. The adder 217 adds together the sampled data for a predetermined time and averages the data that is supplied to the controller 218. The controller 218 analyses the averaged data to monitor the transmission path for faults.
One problem with the conventional OTDR arrangement is that the OTDR signal returned to the OTDR unit in the terminal is typically attenuated by about 30 dB relative to the data signals. This low power level makes subsequent processing and analysis of the signal difficult.
Accordingly, it would be desirable to provide an OTDR arrangement for a multi-span, optically amplified transmission system that overcomes the aforementioned problem.